LENGTH-DISPLACEMENT CHARACTERISTICS OF NORMAL FAULTS ASSOCIATED WITH INCIPIENT CONTINENTAL RIFTING

Length-throw profiles and length (L)–displacement (D) ratios of normal faults of the Okavango Rift Zone (ORZ), Botswana, the youngest branch of the East African Rift System, are investigated using topographic profiles of fault scarps obtained from the Shuttle Radar Topography Mission (SRTM). The ORZ, located between the Congo and Kalahari cratons within the NE-trending Precambrian Damara and Ghanzi-Chobe orogenic belts, forms a half-graben filled by fluvio-deltaic sediments of the Okavango and Kwando rivers. Major normal fault traces parallel the dominant NE trending structural fabric of the basement, defined by compositional layering and foliation produced during isoclinal folding, considered to be responsible for the location and geometry of the ORZ.

Length-throw profiles of individual fault traces show variability in the amount of displacement along the length of the faults. Fault traces vary in length from 65 to 250 km and fault scarp heights vary from 4 to 77m. Several faults (Kunyere, Chobe, Gumare) preserve, to varying extents, parabolic displacement profiles with the maximum fault scarp heights of 42-77m. The Thamalakane Fault preserves a linear displacement profile: maximum fault scarp height of 25m in the SW, decreasing to 8m at the NE terminus. Other faults (Linyanti, Mababe, Phuti) exhibit flat irregular “saw-tooth” profiles with displacements of 4-25m. Log-log plots for these faults show high L/D ratios plotting on or to the right of D_max/L=10^-3. However, L/D data for these faults based on basement throw (Kinabo et al., 2008) show similar or greater displacement than recorded by fault scarps.

Normal faults associated with the ORZ preserve intrinsically long L but lower D_max compared to “young” normal faults. Fault scarp height may record a minimum throw along older faults that affected by aggradation (e.g. Linyanti), degradation (Chobe), or multiple displacement (Kunyere). Fault lengths are less susceptible to these modifications as shown by good agreement between SRTM and geophysical methods. This is consistent with faults associated with the ORZ exploiting the pre-existing weakness of the basement fabric favoring propagation of long faults and leaving increases in displacement for a more evolved rifting stage (see Walsh et al., 2002).